Protection circuits for activated contacts



Dec. 16, 1958 Filed April 25, 1955 J. L. SMITH PROTECTION CIRCUITS FOR ACTIVATED CONTACTS 2 Sheets-Sheet 1 CONTROL CIRCUIT SECTION OF TELfPHOA/E CABLE .34 PEER/7'6 cons v I j 1T CONTROL 3o EFF CIRCUIT /8 SECTION OF l 29 I 27 TLPHONE CABLE [20L FIG. 3 3 v a o IOO- l a 7/ 72 *lM/LHE'NRY com k 80- v 70 i 2 Q z 9.: MIL HENRY COIL k v :g 40- a Q Q 20 /20 MILHNRY COIL THOUSANDS OF co/vracr OPERATIONS INVENTOR V J. L.SM/ TH a WWW ATTORNEY United States Patent PROTECTION CIRCUETS FOR ACTIVATED CONTACTS James L. Smith, Basking Ridge, N; J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 25, 1955, Serial No. 503,768

21 Claims. (Cl'. 317-11) This invention relates to the protection of the switching contacts of' communication, signaling and/or control switching devices such as relays, circuit selecting switches and the like, employed in circuits in which the current in the circuit being closed or opened is normally less than an ampere. A large current for such circuits is, for example, one in the order of eight-tenths ampere.

More particularly, this invention relates to the protection of switching contacts of the above types that must operate in the presence of organic vapors such as those which emanate from numerous types of paints, lubricants and the newer insulating. materials such as cellulose acetate, phenol fiber and the like, being widely used in the manufacture of relays and similar switching devices. Whensaid contacts are operated to open and/ or close current carrying circuits in the presence of even relatively small concentrations of the organic vapors above mentioned, a film of organic material forms on the contact surfaces and the contact surfaces contaminated by such a film are then characterized by those skilled in the art as being activated.

The invention further relates to a convenient method and apparatus for cleaning contacts which have become activated or contaminated as a result of operation in the presence of the above mentioned organic vapors, or contaminated from other causes.

The phenomenon of activation has, for example, been observed to take place when such contacts have been operated in the presence of the organic vapors where the pressure of the organic vapors was only five-thousandths millimeter of mercury.

The public telephone facilities of the United States probably include in the neighborhood of a million switching devices, many of which devices each employ several pairs of switching contacts.

The various pairs of contacts may be assigned various functions. For example, a particular pair of contacts may serve to both open (or break) and to close (or make) a current carrying circuit, or it may be assigned to make only, or to break only, the current carrying circuit, a second pair of contacts being provided to break or make the circuit, respectively, so that the first pair of contacts can perform one or the other of its operations while no current is flowing in the circuit. Still other contacts may perform both of their respective operations while no current flows in the circuit. This last mentioned situation is known in the art as dry operation and involves other considerations than those of immediate interest in connection with the present invention. See, for example, the copending application of T. F. Egan, Serial No. 413,633 filed March 2, 1954 and assigned to applicants assignee. This application matured into Patent 2,812,406 granted November 5, 1957. The brown powder deposit reported by Egan as occurring on contacts of the platinum group of metals used in dry circuits is distinctly different from the film of organic material causing the form of contamination designated activation of contacts used in'wet or current carry- 2,864,976 Patented Dec. 16,1958

ing circuits with which the present invention is concerned. Acc ordingly,,thetwo types of depositsshouldnot be confused. It should be understood, however, that the phenomenon of glow discharge to be described below will also clean contact surfaces upon which abrown powder deposit has been formed.

The normal currents in telephone, switching circuits will range in amplitude between one-tenth and eighttenths ampere. The average current in such circuits can, in general, be assumed to 'be in the order of one half ampere.

To insure. long periods of reliable service and to keep maintenance charges within reasonable bounds, it is necessary, in the majority of cases, that each pair of contacts should be capable of operating a very large number of times without failure or fault. Many such contact pairs, for example, should have a normal life expectancy of several hundred million fault-free operations. For such service conditions it is, obviously, important that all phenomena involving any tendencies toward contact impairment should be most carefully analyzed; Likewise, remedial measures must be carefully gauged and controlled to produce precisely the required degree of compensation for the deleterious propensities of all such phenomena.

In addition to public telephone facilities, numerous other privateand/or governmental communication, signaling, and/or electrical control circuits are being extensively used for which operating conditions closely paralleling those described above are encountered,

The sparking, or discharge, across the gap which occurs at a pair of contacts when the contacts are separated to interrupt or break a current carrying circuit, may involve either or both of'two phenomena which have been designated as arcing"'and glow discharge, respectively.

Because of the phenomenon known in the art as chatter, a pair of contacts is virtually certain to open and close a number of times for each occasion on which they perform either a break or a make operation.

An are, as distinguished from a glow discharge, is a highly localized, high current discharge, requiring only a very modest'voltage' (under twenty volts for most metals). Arcin'g, if sustained for any appreciable interval, results in pitting, mounding (i. e., the formation of pits and mounds on the surfaces of the contacts) and serious erosion and impairment of the surfaces of the contacts. With clean contact surfaces and suitable prior art contact protecting arrangements, however, no appreciable sustained arcing will occur and consequently there will be no serious contact surface impairment as a result of arcing in switching circuits having clean contacts suitably protected by conventional contact protecting arrangements provided that the maximum current is less than one ampere.

A glow discharge, on the other hand, is, as its name implies, diffused and is substantially uniformly distributedover the entire adjacent areas of the pair of contacts. It cantake place where the current is very small, i. e., even as small as one-tenth ampere. or less, but

requires aminimum voltage of approximately 300 volts.

Erosion of the contact surfaces resulting from a glow discharge of limited duration is normally uniform over the entire contact surfaces, particularly where it can, as will be described below, be readily controlled. If unduly prolonged glow discharge may produce a phenomenon known as showering which can result in serious erosion and pitting of the contact surfaces.

As is also pointed out above, the majority of communication circuitsand large numbers, of signaling and control circuits operate with currents of not more than eight-tenths of an ampere, so that, if the contact surfaces are clean and remain substantially clean under 7 conditions for which the circuits were designed. A

typical conventional protection circuit, the use of which will be described more fully below, comprises the se- 1 ries combination of a resistor and a capacitance shunted directly across the contacts to be protected.

As indicated above, however, and particularly with the advent, during recent years, of the use, in telephone switching devices, of newer types of insulating materials such as cellulose acetate in coil windings and phenol fiber for winding-spool heads, a source of serious difficulty, namely a species of contamination by a film of organic material, which those skilled in the art have designated as activation of the surfaces of the contacts, has been encountered to an increasing degree, notwithstanding the fact that the newer insulating materials are in many respects very appreciably superior to those .or closely fitting covers to shield the devices mechanically (particularly from airborne dust) and, in some instances, to afford shielding from electrical or mag netic interference with adjacent apparatus. In some instances, also, it is necessary to employ moisture-proof enclosures.

Such combinations of circumstances result in the early accumulation around the contacts of an appreciable amount of organic vapors and, following a relatively small number of contact operations, say fifty thousand or less) in the presence of these vapors, there develops a coating, or filming over, of the contact surfaces by organic material, which latter condition is designated by those skilled in the art as activation of the contacts.

With the degree of contamination resulting from activation Which usually takes place it has been found that sustained arcing can occur at current values as low as fifty milliamperes, rather than only above the mini mum value of one ampere normally required when the contacts are clean. The pitting, mounding and erosion which results from this low-current arcing encountered across'the activated contacts when breaking a current carrying circuit proves to be substantially as harmful as that caused by arcing at currents in excess of one ampere with clean contacts. The useful life of the contacts is thus very materially reduced.

This explains the fact that although a number of prior art contact protection arrangements have been found entirely effective to protect clean (or inactive) contacts and to enable them to fulfill the requirement of several hundred million fault-free operations, if the contacts become activated, as above described, the normally employed prior art contact protection arrangements appear to hasten, rather than to defer, the deterioration of the contacts. In many instances, for example, activated contacts, with conventional protecting arrangements, fail to function satisfactorily after a relatively small number of operations, such as. y 1011- .sand or less operations.

A discovery upon which the operation of arrangements of the present invention depends is that the above described phenomenon known as glow discharge has a very definite tendency to deactivate or remove the film of organic material and thus to clean the surfaces of contacts which have become contaminated or activated as a result of being operated in an atmosphere which includes organic vapors of the types mentioned above. This discovery, of course, provides a basis for cleaning or deactivating the surfaces of contacts.

Another discovery of importance in connection with the contact protecting arrangements of the present invention is that, for contacts subjected continuously to the conditions which usually produce a harmful degree of contamination or activation, the least erosion takes place if the contacts are maintained in a not quite completely deactivated (or clean) condition.

A principal object of the present invention is to eliminate the deleterious effects which result from the contamination or activation, by organic vapors, of the contacts in communication switching devices.

A further object is to provide a method and means for cleaning and deactivating contact surfaces.

The contact protecting arrangements of the present invention are based upon the above mentioned discovery that where the contact surfaces of communication switching devices are operated under conditions tending to contaminate them, that is to produce the phenomenon known as activation (as also described above), a normal life expectancy of the contacts (several hundred million fault-free operations) can be achieved if at each operation the contact surfaces can be partially, but not quite completely cleaned of the organic film or deactivated.

It has long been known that for current carrying circuits including an appreciable inductive component of impedance, the phenomenon known as glow discharge is likely to occur each time the contacts break the circuit. The duration of the glow discharge is approximately proportional to the current amplitude and the inductive component for the specific circuit being opened. Indeed, one objective in the use of the conventional resistancecapacitance contact protective circuit shunted across the contacts is to convert the impedance effective acrossthe contacts into a virtually resistive impedance and thus eliminate the prolonged glow discharge which the very substantial inductive component, usually contributed by the winding of a switching device being actuated, would otherwise cause.

It should further be borne in mind that, as has been previously mentioned, in view of the phenomenon well known to those skilled in the art as chatter, the large majority of contact pairs of communication switching devices actually open and close numerous times for each individual make or break operation. In view of this, if the circuit is live or wet, i. e., if current flows or is interrupted depending upon Whether the particular pair of contacts under consideration are closed or opened, there is an opportunity for numerous glow discharges to take place whether the particular operation is a make or a break operation. In general, for any specific circuit, the phenomenon of glow discharge, if present, is very much more pronounced for a break than a make operation.

Since, as above described, it has been found that the phenomenon of glow discharge tends to clean or remove the film from activated contact surfaces, the problem here involved is that of causing an appropriate amount of glow discharge to effect the desired degree of contact cleaning or deactivation for each of the contemplated purposes of the invention.

If of suflicient duration the glow discharge will not only completely deactivate the contact surfaces but may also cause showering and serious erosion, which will tend to seriously diminish the number of fault-free bilized at substantially 300 volts.

operations of which the particular pair of contacts is capable.

A specific problem sought to be solved in connection with contact protecting arrangements of the present in vention is that of producing just an appropriate duration of glow discharge at each operation of any particular pair of contacts to effect a not quite complete deactivation of the contacts, since actual tests have indicated that a longer contact life can thus be realized where the contacts are to be operated under the conditions which tend to contaminate or activate them. From the standpoint of a particular circuit, under specified operating condi tions such as normal or average current and break only, make only or both break and make requirements for the particular pair of contacts, the problem usually resolves itself into the question of how much inductive reactance should be added closely adjacentto the contacts and in series therewith to produce a duration of glow discharge just sufiicient to effect the desired, not quite complete, deactivation of the contacts at each operation. In the case of the majority of contaminated or activated contact conditions encountered in telephone switching circuits, a glow discharge duration in the order of one half microsecond has been found to produce excellent results.

where L is the inductance in henries of the cleaning or deactivating coil placed immediately in series with the active contacts, R is the protective circuit resistance in ohms, I is the current in amperes, and V is the glow discharge voltage. Since flow discharge is a constant current density phenomenon the voltage V will be sta- It is further assumed that the value of resistance R will lie between the quantity I V e (where C is the capacity in microfarads of the protective circuit'capacitance) and 300 ohms.

By way of a specific numerical example, if L is assumed to be one millihenry, R 100 ohms, and C one half microfarad, the glow discharge duration t will be 1.6 microseconds. This is substantially three times too long for optimum results in the case of contacts which break the switching circuit and the value of L should therefore be reduced for such circuits to product a duration t of substantially five-tenths microseconds. In a majority of telephone switching circuits in which the contacts break the switching circuit, a value of L between 100 and 200 microhenries has been found to produce optimum re sults.

Where the closed-circuit current in the particular circuit under consideration is relatively large, for example, from five-tenths to eight-tenths of an ampere, or is subject to amplitude variations over a wide range of values, the inductance or retardation coil provided to induce the desired degree of glow discharge is, in accordance with the principles of the invention, preferably a re- :tardation coil having a core of initially 'highzperrneability which, however, becomes saturated .at a low value of current through the coil. Under such circumstances the coil should be designed so that the core will reach a condition of magnetic saturation at a predetermined low value of current in the circuit. A preferred material for the core of such a coil is, for example, a ferrite such as nickel-zinc ferrite.

Such a coil will, because of the high permeability of its core prior to becoming saturated, effectively retard the initial build-up of current upon closure, thus reducing difficulties resulting from the chatter of the contacts as well as producing the required glow discharge. Furthermore, as the core becomes saturated its effective inductance is reduced to a negligible value thus limiting the duration of the glow discharge to the time interval required to effect the desired degree of deactivation and avoiding the deleterious effects of a too prolonged glow discharge. Such a coil will operate satisfactorily over a sufficiently wide range of current values that only a few designs will suffice to cover substantially all telephone switching circuits. The necessity of providing a large number of different coil designs to include one for each current value of each particular circuit is thus avoided. Also, the use of a saturable core eliminates difficulties arising from the fact that the chatter openings during a make operation takes place while the current is building up so that an air-core coil of suitable inductance for the fully established circuit current may prove inadequate during the chatter openings.

In connection with the problem of cleaning or deactivating contact surfaces which havebecome activated to an objectionable degree, it will become apparent, during the course of the detailed description given hereunder of specific illustrative arrangements of the invention and of experiments conducted to discover the behavior of activated contact surfaces under particular conditions, that complete cleaning of the activating 'film from contact surfaces is merely a matter of subjecting them to a suitably controlled glow discharge cleaning process.

These and other features, objects and advantages of the arrangements of the present invention will become more readily apparent during the course of the following .detailed description of preferred illustrative embodiments of the invention shown in the accompanying drawings and fromthe appended claims.

Fig. 1 illustrates in schematic diagram form arepresentative communication switching circuit modified in .accordance with the principles of the present invention to avoid the deleterious elfects resulting fromthe tendency of the contact surfaces to become contaminated or activated,

Fig. 2 is a circuit of the type illustrated in Fig. 1 but in which the contact cleaning coil is provided with a core of initial high permeability, the core becoming 'saturated at a low value of current;

Fig. 3 shows curves of arc duration in microseconds versus thousands of contact operations for activating and deactivating cycles of three diiferent pairs of contacts;

Fig. 4 illustrates curves showing contact erosion versus cleaning coil inductance L, for circuits of the types illustrated in Figs. 1 and 2; and

Fig. 5 illustrates a further type of circuit embodying the principles of the present invention.

In more detail, in Fig. 1 a common form of circuit employed in telephone systems, but equipped witha cleaning coil or inductance of the invention, is illustrated in schematic diagram form.

The circuit of Fig. 1 comprises the following: A battery 8, which has, in a typical case, a voltage'of'48 volts; at current limiting resistance 10 having in a typical case a value of 'ohrns; a low resistance, high operating velocity, relay 12, the winding of which has, in a typical case, a resistance 14 of l'6ohms and an inductance :16

the respective pairs of contacts. 'which even the smallest of these cleaning coils deacti- 'of approximately one henry; a section 18 of telephone immediately in series with contacts 28, 30, and comprising an air-core coil having an inductance of 100 .microhenries or somewhat more, as will be explained in greater detail hereinunder; and a pair of contacts 28, 30,

which upon closure complete a direct current circuit and cause current from battery to operate relay 12 in conventional manner.

The contacts 28, 30, in turn, are a pair of relay contacts operated by a relay winding 29, which winding is connected by leads 27 to a control circuit 37, from which control circuit current to cause closure of contacts 28, 30,

is furnished.

Contacts 28, 30, and their associated winding 29, are,

.further, within an enclosure 36, within which enclosure there may also be one or more other switching devices or other apparatus (not shown), the windings of all of which switching devices and apparatus utilize insulating materials, paints or lubricants which emit organic vapors in sufiicient quantity to build up within said enclosure an appreciable concentration of said vapors.

In the absence of suitable protection such as that afforded by coil 26 in accordance with the principles of the present invention, these vapors will cause contacts 28, .30, to become activated," that is to become coated with a film of organic substances deposited from said vapors,

after said contacts have operated in the order of twenty thousand times or more in the presence of said vapor.

In Fig. 3 are curves illustrating the activation and deactivation of three separate pairs of contacts in terms of arc duration in microseconds versus thousands of contact operations.

In each instance the pair of contacts was employed to break or interrupt a current carrying circuit of the type illustrated by Fig. 1, except that in each instance, for operations from zero to 100,000, curves 70, 72 and 74, respectively, the contacts had no protecting or cleaning coil such as 26 of Fig. 1 connected directly in series with them. Also for operations from 100,000 to 150,000, the cleaning coil corresponding to coil 26 of Fig. 1 was given inductance values of one milhenry, 9.5 milhenries and 120 milhenries for curves 71, 73 and 75, respectively.

The are duration in microseconds is substantially proportional to the degree of activation.

Curve 74 represents the process of becoming activated starting with a pair of contacts which are virtually clean or completely deactivated.

Curves 70 and 72 represent the process of activation starting with contacts not quite completely deactivated, curve 70 representing a slightly greater initial activation than curve 72.

As illustrated by curves 70, 72 and 74, the arc duration in microseconds increases very moderately during the first ten thousand or so operations but thereafter increases much more rapidly, so that at fifty thousand operations all three pairs of contacts are definitely activated to an objectionable degree. At one hundred thousand operations all three of the curves 70, 72 and 74 indicate that a saturated or maximum degree of activation is being approached, and cleaning or deactivating coils, having, as above stated, values of one milhenry for curve71, 9.5 milhenries for curve 73, and 120 milhenries for curve 75, are then inserted immediately in series with The rapidity with vates its associated pair of contacts is strikingly illustrated by the curves 71, 73 and 75, so that after fifty thousand more operations all of the contact pairs have 8 been reduced to a degree of activation such that arcing of any significant duration no longer occurs. Arc durations were observed by connecting one pair of deflecting plates of an oscilloscope across the contacts and timing the sweep to coincide with the period during which the arcing, if any, occurred.

The salient features of the operation of the circuit of Fig. 1 are as follows: A few milliseconds after closure of contacts 28, 30, a direct current of approximately one half ampere will flow through the series circuit including battery 8, resistance 10, the winding of relay 12, the section of telephone cable 18, the cleaning coil 26 and said contacts. Thereafter, when contacts 28, 30 open, the cleaning coil 26 will cause a momentary voltage surge across contacts 28, 30, in excess of 300 volts, which in turn will cause a momentary glow discharge having a duration in the order of one half microsecond, to take place between contacts 28, 30. The glow discharge will tend to largely remove the contaminating or activating coating or film which would otherwise form on the surfaces of the contacts 28, 30.

In Fig. 4 are shown the results of a series of experiments, illustrating erosion per operation versus cleaning coil inductance, with a circuit of the type illustrated in Fig. l, in which the cleaning coil 26 Was successively given values of inductance from 50 to 1,000 microhenries. The anode is, of course, the contact connecting to the positive terminal of the battery 10 which, in Fig. 1, is contact 28. The cathode is, of course, the opposing contact 30, of Fig. 1. As is apparent from the portions 52, 53 and 62, 63 of the curves 50 (anode) and 60 (cathode), respectively, the optimum range of values for the inductance ofthe cleaning coil 26 of Fig. 1 is between to 200 microhenries, though reasonably low erosion is still obtained with the maximum value of 1,000 microhenries. Other experiments have indicated that any further substantial increase in the inductance of the cleaning coil, in circuits in which the contacts function to break the circuit, would so prolong the duration of the glow discharge that appreciable showering would be encountered and pitting and excessive erosion of the contact surfaces would be likely to result. The portions 54 and 64 of curves 50 and 60, respectively, illustrate graphically that for values of cleaning coil inductance less than 100 microhenries, the erosion of both contacts increases sharply with decreasing inductance.

Other experiments demonstrated that the duration of the glow discharge produced by the cleaning coils within the optimum range of 100 to 200 microhenries of the inductance values was in the order of one half microsecond.

Still other experiments demonstrated that as the value of inductance of the cleaning coil was increased toward 1,000 microhenries the duration of the glow discharge increased toward a maximum in the order of 1.6 microseconds at 1,000 microhenries.

It was further found that, while a cleaning coil having an inductance in the order of 1,000 microhenries appeared to substantially clean or deactivate the contacts, erosion (having the descriptive designation mound-pit erosion) could be detected by microscopic examination and that premature failures of the contacts by mechanical locking were likely to be encountered after a relatively moderate number (less than one million) of operations.

As contrasted with the above, for cleaning coils having an inductance within the preferred or optimum range of 100 to 200 microhenries, some residual contamination or activation of the contacts was always found present upon microscopic examination and substantially uniform and relatively very moderate erosion or wear over the entire contact surfaces occurred. Contacts so protected were found capable of several hundred million performed by another pair of contacts.

operation.

9 fault-free operations in the presence of an appreciable concentration of organic vapor equivalent 'to or exceeding that encountered under normal present day operating conditions.

Experiments with other circuits of the general type illustrated in Fig. l, but having individual circuit element values such that their normal closed-circuit currents were appreciably greater or smaller than for the specific illustrative circuit described in detail above, clearly indicated that the inductance of the cleaning coil 26 should, in general, be such that a glow discharge having a duration of one half microsecond will be obtained. Contacts so protected are capable of fault-free operation for several hundred million operations in any concentration of organic vapors likely to be encountered in service.

A further problem in contact protection is posed by circuits in which the particular pair of contacts being considered is required to perform the operation of closing the circuit only, another pair of contacts beingemployed to break the circuit prior to separating the particular pair of contacts for a subsequent closing operation. In such a case the normally unwanted openings resulting from the inevitable chatter of the contacts must, as mentioned hereinabove, be relied upon to produce the desired degree of cleaning or deactivation at each closing operation of the particular pair of contacts. In general for contacts required only to make or complete the current carrying circuit, a cleaning inductance very appreciably larger than that required for contacts which ae employed tobreak the circuit will be found necessary to effect the desired degree of cleaning or deactivation. 'For example, where, in a circuit of the type illustrated in Fig. 1, a cleaning inductance of from 100 to 200 microhenries will be found satisfactory for contacts employed to break or open the circuits while normal current is flowing through the contacts, an inductance in the order of ten milhenries is necessary to satisfactorily protect contacts employed only to complete the circuit and to re-establish the flow of current.

In Fig. 2, a circuit identical with that of Fig. 1 except that in Fig. 2 the contact cleaning coil 32 is provided with a core of a ferrite, such as nickel-zinc ferrite, which at low flux densities has a high permeability, i. e., in the order of 4,000, but which becomes saturated at a low current value, such as one-hundredth ampere, whereupon its effective inductance drops to a small, substantially insignificant, value. With this type of cleaning coil a suitable inductance at low current values is always present so that the circuit will function satisfactorily over a wide range of closed-circuit current values. The relatively high inductance at low current values also functions to delay or retard the initial current surge at the instant of closing and will provide the required duration of glow discharge as the circuit is opened and the current drops below the saturation value of the core.

The ferrite core, contact cleaning coil is, by way of example, also of outstanding value in the problem case described above in which the particular pair of contacts under consideration are required only to make the circuit (i. e., to complete the current carrying path) and the function of breaking the current carrying path is For the particular pair of contacts under consideration, in such a case, reliance for contact cleaning must be placed upon the inevitable and normally unwanted openings caused by chatter of the contacts during the circuit closing The ferrite core coil will, in this instance, provide a suitably large inductance during the unwanted openings which occur at low current values (i. e., as the current begins to build up) but assoon as the current has reached saturation value in the coil, only an insignificant value of inductance will remain. However, 'sufficient energy will have been stored in the coil to provide adequate glow discharge for any subsequent unwanted openings. The effective duration of glow discharge for a circuit closing operation of the contacts can in this way be suitably controlled regardless of the amplitude which the current finally reaches when the contacts, have ceased chattering and the circuit is completely established.

It should be noted that the operation of the contact cleaning coils of the present invention is readily distinguishable from that of very small inductances (from one to ten microhenries) used in the same circuit position in prior art contact protection arrangements, for example see United States Patent 2,594,890 granted April 29 ,1952, to W. B. Ellwood.

As is obvious from the curves of Fig. 4 of the drawings accompanying the present application, coils of such small inductance are incapable of producing a glow discharge of sufficient duration to effect the necessary degree of cleaning of the activated contacts. The function of such a small inductance is merely to lessen the surge of current upon initial contact closure which tends to occur when the load circuit to which the contacts are connected has appreciable distributed capacity and becomes charged when the contacts are open. This accumulated charge, of course, tends to discharge through the contacts as they approach closure.

Conversely the larger contact cleaning inductances of the present invention will, obviously, effectively prevent the above described initial surge resulting from discharge of the energy stored in the distributed capacity of the circuit, as well as produce the appropriate duration of glow'discharge required for the desired partial deactivation of the contacts in accordance with the present in vention.

In Fig. 5 a further circuit of the invention is illustrated. As indicated by the use in Fig. 5 of many of the designation numbers employed in Figs. 1 and 2, a majority of the elements of Fig. 5 can be identical with those of Figs. 1 and 2 having the corresponding designation numbers, respectively, and can be as described in detail above in connection with the last mentioned figures.

The circuit of Fig. 5 differs from that of Fig. 1, however, in that the load 9, which can, for example, be a signal lamp, has a substantially purely resistive impedance 15 of, for example, 400 ohms. having a purely resistive load, the conventional contact protective circuit comprising resistance 22 and capacitance 24 of Fig. 1 can be omitted and coil which can be similar to coil 26 of Fig. 1, or alternatively similar to coil 32 of Fig. 2, can constitute the sole protective element for contacts 28, 30.

In some instances a particular pair of contacts may be required to operate only upon rare occasions, as, for example, where the contacts are included in a spare or reserve unit. Also there are instances where th unit may be stored for a long period before it is placed in service. Under such conditions the contacts may become contaminated or dirty and the unit may fail to operate properly when it is eventually placed in service. The contacts of all such units can periodically be effectively cleaned by controlled glow discharge, utilizing, for example, a circuit of the type illustrated in Fig. 5, where the unit whose contacts are to be cleaned is connected as shown for contacts 28, 30, and actuating coil 29. Control circuit 37 can be any conventional source of appropriate control pulses such, for example, as a multivibrator circuit. Control circuit 37 is then operated to provide substantially fifty thousand successive operating pulses and the process of cleaning by glow discharge will have then been completed. For such cleaning operations coil 80 should preferably have an inductance between 200 and 1,000 microhenries. Stated in terms of glow discharge duration following each break operation, an inductance coil causing a glow discharge of between one half and one and six-tenths microseconds With a circuit duration, should be employed for cleaning contacts which have become contaminated from any of the above mentioned conditions.

Numerous and varied other arrangements embodying the principles of the present invention and within the scope ofv the inventive concepts involved will readily occur to those skilled in the art. It is, accordingly, to be clearly understood that the specific embodiments described above are illustrative only and are not intended to exhaustively cover all possible arrangements in accordance with the present invention.

What is claimed is:

1. Means for preventing a harmful degree of contami nation in the presence of organic vapors of switching contacts employed to interrupt the current flow in a switching circuit, said circuit presenting an impedance across the contacts which is virtually resistive and carrying a maximum current of less than one ampere, said means comprising solely an inductance having a value greater than 100 microhenries and less than 1,000 microhenries connected directly in series with said contacts, the combination of said inductance and said contacts being connected directly across the remainder of said switching circuit, said inductance comprising the sole electrical connection between one of said contacts and the remainder of said circuit.

2. Means for preventing harmful contamination in the presence of organic vapors of switching contacts employed to interrupt the current flow in a switching circuit, said circuit presenting an impedance across the contacts which is virtually resistive and carrying a maximum current of less than one ampere, said means comprising solely an inductance connected directly in series with said contacts, said inductance having a value not exceeding 1,000 microhenries but sufiicient to produce a glow discharge across said contacts when interruping said current flow in said switching circuit, the combination of said inductance and said contacts being connected directly across the remainder of said switching circuit, said inductance comprising the sole electrical connection between one of said contacts and the remainder of said circuit, said glow dischage having a duration between one half microsecond and one and one half microseconds.

3. An inductance coil for cleaning switching contacts for opening and closing a circuit presenting an impedance across the contacts which is virtually resistive and carrying a maximum current of less than one ampere, said contacts operating in an appreciable concentration of organic vapors, said inductance coil having a core of a ferrite which has a large permeability at low current values through said coil but becomes saturated when currents in excess of one-hundredth ampere are passed through said coil, said coil being connected directly to one of said contacts and comprising the sole electrical connection between said contact and the remainder of said circuit, said coil having a low current inductance of not less than 100 microhenries and a high current inductance of not greater than 1,000 microhenries.

4. A device for protecting switching contacts from contamination when operating in the presence of organic vapors, said contacts being arranged to open and close a circuit which presents an impedance across the contacts which is virtually resistive, said circuit carrying a maximum current of less than one ampere, said device comprising an inductance coil having a core of a material having high permeability at low values of current through said coil, said core becoming saturated at a current of less than one-tenth ampere through said coil, said coil being connected directly to one of said contacts and comprising the sole electrical connection between said contact and the remainder of said circuit, said coil having a low current inductance of not less than 100 microhenries and a high current inductance of not greater than 1,000

microhenries.

5. In a switching circuit carrying a maximum current of less than one ampere, contact protecting means for the switching circuit contacts for opening and closing said circuit in the presence of an appreciable concentration of organic vapors, said means comprising solely in combination, an inductance coil having an inductance between and 1,000 microhenries connected directly in series with said contacts and comprising the sole electrical connection between one of said contacts and said circuit, and a second circuit directly shunted across said series circuit comprising said contacts and said coil, said second circuit comprising a resistance and a capacity connected in series, the remainder of said switching circuit being shunted by said combination, the combined impedance of said remainder of said switching circuit and said second circuit being virtually resistive.

6. The combination of claim 5 in which said coil has a core of a material having a high initial permeability, said core becoming saturated at a current of less than one-tenth ampere through said coil.

7. An electrical circuit the maximum operating current of which is less than one ampere, said circuit comprising in series a source of direct current energy, a current limit ing resistance, the electrical winding of an electromagnetic switching device to be controlled by said circuit, a section of transmission line, a pair of contacts to open and close said circuit, said pair of contacts beingenclosed in an atmosphere which includes an appreciable concentration of organic vapor, said concentration of vapor being sufiicient to contaminate said contacts after fifty thousand operations or less, and contact protective means comprising solely the combination of a contact cleaning coil connected immediately adjacent and in series with said pair of contacts in said circuit, said coil having an inductance within the range of 100 to 200 microhenries, and an impedance corrective circuit directly shunted across the series combination of said contacts and said cleaning coil and the end of said transmission line nearer said contacts, said corrective circuit comprising a resistor and capacitor connected in series, said corrective circuit rendering the. impedance at the said nearer end of said transmission line virtually resistive, whereby when said contacts open said circuit a glow discharge is produced of sufficient duration of partially clean said contacts and substantially inhibit erosion of said contacts.

8. The circuit of claim 7 in which said cleaning coil has a ferrite core.

9. An electrical communication circuit having a maximum operating current of less than one ampere, said circuit comprising a source of direct current energy, a load impedance, a pair of contacts adapted and connected to open and close a direct current path including said source and said impedance, said contacts being immersed in an atmosphere having a sufiicient concentration of organic vapor to contaminate said contacts, and contact protective means comprising solely the combination of a cleaning coil connected directly in series with said contacts the inductance of said coil being suflicient to produce a glow discharge having a duration of one half microsecond when said contacts are opened, and an impedance corrective network directly shunting the series combination of said cleaning coil and said contacts and the remainder of said circuit, the combination of said corrective network and said remainder of said circuit presenting an impedance which is virtually resistive.

10. Means for cleaning contaminated contacts of an electromechanical switching device, said means comprising solely the combination of an inductance coil having an inductance of between 200 and 1,000 microhenries connected directly adjacent to and in series with said contacts, means for establishing a substantially purely resistive circuit in shunt relation across the series connected combination of said contacts and said coil in which resistive circuit a direct current of substantially one half ampere flows when said contacts are closed, and means 13 for operating said switching device to open and close said contacts successively fifty thousand times.

11. Contact cleaning means comprising solely an inductance directly connected in series with the contacts, the inductance having a ferrite core, said core having a large permeability at low current values but becoming saturated at current values in excess of one-hundredth of one ampere.

12. Means for preventing harmful contamination by organic vapors of switching contacts employed to close a switching circuit to establish the flow of current therethrough, the maximum current in said circuit being less than one ampere, said means comprising solely an inductance connected directly in series with said contacts, said inductance being sufiicient to cause a glow discharge of substantially one half microsecond duration between said contacts during their chatter open periods, the combination of said inductance and said contacts being connected directly across the remainder of said circuit, said inductance comprising the sole electrical connection between one of said contacts and the remainder of said circuit, the remainder of said circuit having an impedance which is virtually resistive.

13. The means of claim 12 in which said inductance has a core of a material having a large permeability at low current values but becoming saturated at current values in excess of one-hundredth of one ampere.

14. Contact protecting means for protecting the contacts which open and/or close a switching circuit which includes the actuating coil of an electromagnetic relay, the maximum current in said switching circuit being less than one ampere, said means comprising solely in combination a first circuit shunted across said switching circuit at a point closely adjacent said contacts, said first circuit including a resistance and a capacitance connected in series, and a second circuit shunted across the same point of said switching circuit, said second circuit comprising an inductanceconnected directly in series with said contacts, said inductance having a value of between 100 and 200 microhenries, said inductance comprising the sole electrical connection between one of said contacts and the remainder of said circuit, the remainder of said circuit having an impedance which is virtually resistive.

15. The combination of claim 14 in which said inductance has a core which has high permeability for low current values through said coil but becomes saturated at current values through said coil exceeding one tenth ampere.

16. Contact protecting means for protecting the contacts which open and/or close a switching circuit having a. virtually purely resistive impedance, said means comprising solely an inductance coil connected directly in series with said contacts, said coil comprising the sole electrical connection between one of said contacts and the remainder of the circuit and having a value of inductance between and 1,000 microhenries.

17. The protecting means of claim 16 in which said inductance coil has a core having high permeability at low values of current through the winding of said coil and substantially unity permeability at current values through the winding of said coil which exceed one-tenth ampere.

18. Contact protecting means for protecting the contacts which open and/or close a switching circuit in the presence of organic vapors, said circuit having a virtually purely resistive impedance, said circuit carrying a maximum current of less than one ampere, said means comprising solely an inductance coil connected directly in series with said contacts, said coil comprising the sole electrical connection between one of said contacts and the remainder of the circuit and having a value of inductance between 100 and 10,000 microhenries.

19. The protecting means of claim 18 in which said inductance coil has a core having high permeability at low values of current through the winding of said coil and substantially unity permeability at current values through the winding which exceed one-tenth ampere.

20. The method of protecting a pair of contacts in a switching circuit carrying a current of less than one ampere and operating in the presence of organic vapors, said method comprising modifying the impedance of the circuit to be controlled by said contacts to present a virtually purely resistive impedance at said contacts and inserting in series relation between one of said contacts and said modified circuit an inductive impedance sufficient to produce a glow discharge across said contacts of substantially one microsecond duration whereby excessive contamination of the surfaces of said contacts is inhibited.

21. Means for cleaning contacts of switching circuits carrying a maximum current not exceeding one ampere, which contacts have become contaminated by organic vapors or contaminated from other causes, said means comprising a virtually resistive circuit connected to be controlled by said contacts, said circuit, when closed by said contacts, carrying a maximum current of less than one ampere, means for causing a glow discharge of substantially one microsecond across said contacts during a break operation of said contacts, and means for operating said contacts fifty thousand times.

References Cited in the file of this patent UNITED STATES PATENTS Germer Aug. 6, 1957 

